1. Field of the Invention
The present invention relates to a composition containing a photoacid generator monomer, a solid substrate coated with the composition, a method for synthesizing a compound on a substrate using the composition, and a microarray produced according to the method.
2. Description of the Related Art
Methods for synthesizing a biopolymer on a substrate have been well known for years. In one example, Fordor et al. teaches a new synthesis technique where nucleic acid or amino acid having an UV-labile protecting group is attached on a solid surface. The protecting group is removed by exposing the selected regions of the solid surface to light using a photolithographic mask, which is subsequently reacted with a new nucleic acid or amino acid having a photolabile protecting group, to polymerize the nucleic acid or amino acid at a specific location (see U.S. Pat. Nos. 5,445,934 and 5,744,305). Since this method allows selective synthesis of oligonucleotide probes with a specific sequence/length at a specific location, it is useful in synthesizing various oligonucleotide probes with a desired sequence and length at a predetermined position. Also, since this method employs an ultra-fine processed mask used in semiconductor devices, it is extremely useful for fabricating oligonucleotide probes in high density. Fordor et al. also suggested that a sequencing method using the oligonucleotide probes, which is much easier and faster than Sanger's method, is useful for making high-density oligonucleotide probes. However, the removal of a photolabile protecting group is proportional to the power of a light source, which plays a detrimental role in the ultra-fine process for making high-density chips.
On the other hand, photolithographic process using photoresist (hereinafter referred to as ‘PR’), which is used for micropattern formation in the semiconductor industry, has attracted attention as an essential technique to improve the density of a DNA chip. Since the size or capacity of a semiconductor chip is dependant on the spatial resolution of the photolithographic process, such process has played a leading role in the semiconductor and microelectronics industry. Photolithographic process utilizes the solubility differences of PR between the light-exposed region and the unexposed region. A solubility reduction in the exposed region is referred to as a negative system, and a solubility increase is referred to as a positive system and is used mostly for the production of semiconductor chips. By using the above photolithographic process, more oligonucleotide probes can be arrayed on a limited chip area. Up to now, the photolithographic process has been applied in a method using a general PR system (see U.S. Pat. No. 5,658,734) and a method using a micromirror. Photolithographic process using a PR system (hereinafter referred to as a ‘PR process’) has an advantage of using materials already developed or commercialized for the semiconductor industry. According to the process, a pattern is formed by the light exposure, washed out to lead to standard solid-phase nucleic acid synthetic reaction on the surface, and finally linked to nucleotides. The PR includes diazoquinone/cresol-novolac, highly adhesive to a surface, which is shown to have good pattern characteristics in i-line (365 nm) and is used in 16 megabyte DRAM processing. However, the PR is developed in an alkaline solution ([OH−]>0.1M), which causes the cleavage of an amide bond protecting the amino group of base. The protecting coating under PR has been suggested to overcome the said problem of development (see J. Vac. Sci. Tech., B7(6):1734, 1989).
The PR process consists of three major steps. The first step is mainly the PR pattern formation by PR coating, light exposure, and developing. The second step is the removal of a protecting group in the exposed and etched region and the PR by acidic solution. The third step is the sequential attachment of nucleic acid and a post-treatment. The PR process has a disadvantage of complex processing as described above.
To overcome this downside of the PR process, a photoacid patterned array (PPA) system had been proposed (see U.S. Pat. No. 5,658,734). The PPA system uses a polymer matrix mixed with a photoacid generator (hereinafter referred to as ‘PAG’). In this PPA system, acids are generated only at the exposed region, and the removal of a protecting group occurs after heat treatment. Therefore, the two separate steps employed in the PR process can be carried out in one step. However, this PPA system has revealed several problems as well. For example, acids generated by the PPA system remain in the polymer matrix, and more PAG should be added. An excess amount of PAG scatters light, which in turn interferes with micro pattern formation. Furthermore, the polymer matrix (a film of a mixture of polymer and PAG) has to be removed using an organic solvent such as acetone or methylethylketone (MEK), for which reprocessing and recovery is expensive.
Another method for manufacturing a DNA chip using photolithography is one using a micromirror. This method comprises coating on a solid substrate using PR which is capable of reacting with oligonucleotides, adding a solution of a photoacid generator to the wall, and exposing a predetermined portion to light using a micromirror to generate acids. Thus, the generated acids may remove a protecting group attached to the oligonucleotides, thus allowing the reaction of the oligonucleotides. Repeating the above steps allows layering oligonucleotides in a desired pattern. This method for manufacturing a chip is simple compared with the PR method. But, this method has disadvantages: the reaction of the photoacid generator is carried out in solution, thus resulting in a difficulty in manufacturing a high density chip, and the cost of equipment is high.
To overcome the problems of the prior art, U.S. Pat. No. 6,359,125 discloses a method for preparing arrays of peptide nucleic acid (PNA) on a solid substrate using a polymeric photoacid generator, in place of the photoacid generator in a polymer matrix. However, the use of a polymeric photoacid generator, which involves a further step of polymerizing monomers, results in a high production cost and complicated processes. Thus, there is still a need for a photoacid generator which can be easily produced at low cost using conventional methods and has high acid-generating efficiency. The present inventors conducted research to overcome these problems and discovered a photoacid generator monomer of the present invention.